13.3 Make the earth move

Saprotrophic fungi can be physically destructive even without digesting
things. Mushrooms may be soft and squashy, but they have been known to lift
stone slabs and force their way through tarmac. In the 1860s a famous mycologist
called Mordecai Cubitt Cooke wrote A Plain and Easy Account of British Fungi
(Cooke, 1862) in which he told of:

“...a large kitchen hearthstone which was forced up from its bed by an
under-growing fungus and had to be relaid two or three times, until at last it
reposed in peace, the old bed having been removed to a depth of six inches and
a new foundation laid.”

Cooke also tells of a comparable observation made by a Dr Carpenter:

“...Some years ago the town of Basingstoke was paved; and not many months
afterwards the pavement was observed to exhibit an unevenness which could not
readily be accounted for. In a short time after, the mystery was explained,
for some of the heaviest stones were completely lifted out of their beds by
the growth of large toadstools beneath them. One of the stones measured
twenty-two inches by twenty-one, and weighed eighty-three pounds..."

It’s not only 19th century structures that are prone to fungal attack. The
BBC News website of 30th October 2008 carried a report about mushrooms lifting
the tarmac driveway of a house in Reading, Berkshire (Fig. 1).

Another interesting pictorial example is on the back cover page of the April
1991 issue of the Mycologist, a magazine published by the British
Mycological Society. This photograph shows fruit bodies of the puffball
Scleroderma bovista coming through a tennis court. In this case the
constructional history was recorded as follows:

"the original hard porous court made of fly ash was overlaid in 1989 with
75 mm of gravel and then a 20 mm layer of tarmacadam was rolled smooth over
the top (Taylor & Baldwin, 1991). The first fruit bodies appeared in 1990
(Fig. 1B)."

Buller (1931) did some experiments in which he put weights on the top of
developing mushrooms to see how much pressure they could exert (Fig. 2). He
worked out that a single mushroom could apply a pressure of at least two thirds
of an atmosphere - that is about ten pounds per square inch. It’s all a matter
of hydraulics, of course; as we indicated in Chapter 6 (in our ‘hydrostatic
bricks’ discussion; CLICK HERE
to view the page), the mushrooms can fill themselves with water and force their
way through cracks and crevices. They are not doing it because of some perverse
intention to break up paving, but because in nature they need to push through
soil and plant litter in order to bring their fruit bodies to a position from
which they can release their spores to the breeze; if there happens to be a
tennis court in the way, then game, set and match to the fungus!

Fig. 2. Diagram of an experimental rig (original appears as Fig. 67 in
Buller, 1931) to test the ability of fruit bodies of Coprinus
sterquilinus to lift quantities of lead shot. The Coprinus
fruit body (C) had grown on some balls of horse dung (B)
incubated in a glass dish (A) in the laboratory. A glass
tube (H) was placed over the fruit body to stabilise it
with clamp (I) and then the fruit body cap (D)
was covered with a small glass beaker (E) and then loaded
with a test tube containing lead shot (F) to a total weight
of 150 g. The fruit body grew a further millimetre in 2 h after this and
even the addition of a further 50 g weight (G) did not
decrease the rate of growth. Only when the overall loading was increased to
300 g did the fruit body stem (C) bend and break.